Convergent close-coupling approach to positron-helium collisions
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This thesis presents the theoretical studies of positron scattering on the helium atom using two-center Convergent Close-Coupling (CCC) theory.The thesis organized in the following way: In the Introduction (Chapter I) the motivation for the study and the current status of the positron-helium scattering problem are presented. Other theoretical methods that previously have been applied to this problem are reviewed and their limitations are indicated. The application of the two-centre CCC to positron-helium is presented in Chapter 2. The derivations of the scattering equations and transition matrix elements are given in detail. The results of the two-centre CCC calculations for positron scattering from the helium ground state are compared with available experimental data and the results of other calculations in Chapter 3. Chapter 4 presents CCC results of positron scattering from the metastable 2[superscript]3S excited state of helium. Finally, in Chapter 5, we draw conclusions arising from this work and indicate future directions for the research.Main results of this work • The two-centre Convergent Close-Coupling method is generalized to positron scattering on the helium atom. • Cross sections, free from pseudo-resonances, are obtained for all major channels of interest and over a wide range of scattering energy. The importance of the inclusion of pseudo-states from both centers is demonstrated. • The elastic cross section below the Ps-formation threshold is obtained, for the first time, by using a two-centre close-coupling approach. • A consistent agreement of the CCC calculations with the available integrated cross section measurements of Ps-formation has been achieved at energies from the threshold of 17.8 eV up to 150 eV.• The CCC calculations exhibit a qualitative agreement with the measurements of the total cross section for ionization of the ground state of helium by positron impact. • The separation of the spin part of the total wave function is demonstrated and possible ortho- to para- Ps formation ratios are derived from first principles without assumptions. • Calculations of positron scattering from the 2[superscript]3S metastable state of helium are done for the first time. Converged results for the total scattering, Psformation, helium and Ps excitation as well as breakup cross sections are obtained. • Both considered cases (scattering from the ground and the 2[superscript]3S metastable states) confirm the equal atomic and Ps contributions to near-threshold breakup cross section, a behavior previously observed in positron scattering from hydrogen.
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Abdurakhmanov, Ilkhom (2011)This thesis is devoted to extension of the convergent close-coupling (CCC) method to heavy projectiles and its application to the theoretical studies of antiproton scattering on the hydrogen and helium targets.In the ...
Bray, Igor; Bailey, J.; Fursa, D.; Kadyrov, A.; Utamuratov, R. (2016)The positron-atom scattering problem contains the rearrangement channel of positronium (Ps) formation. While this makes the problem particularly difficult to calculate, it has the unusual benefit of validation via ...
Zammit, M.; Fursa, Dmitry; Savage, J.; Bray, Igor; Chiari, L.; Zecca, A.; Brunger, M. (2017)© 2017 American Physical Society.The single-center adiabatic-nuclei convergent close-coupling method is used to investigate positron collisions with molecular hydrogen (H2) in the ground and first vibrationally excited ...